drm/xe/madvise: Track purgeability with BO-local counters

drm/xe/madvise: Track purgeability with BO-local counters

xe_bo_recompute_purgeable_state() walks all VMAs of a BO to determine
whether the BO can be made purgeable. This makes VMA create/destroy and
madvise updates O(n) in the number of mappings.

Replace the walk with BO-local counters protected by the BO dma-resv
lock:

  - vma_count tracks the number of VMAs mapping the BO.
  - willneed_count tracks active WILLNEED holders, including WILLNEED
    VMAs and active dma-buf exports for non-imported BOs.

A DONTNEED BO is promoted back to WILLNEED on a 0->1 transition of
willneed_count. A BO is demoted to DONTNEED on a 1->0 transition only
when it still has VMAs, preserving the previous behaviour where a BO
with no mappings keeps its current madvise state.

PURGED remains terminal, preserving the existing "once purged, always
purged" rule.

Fixes: 4f44961eab84 ("drm/xe/vm: Prevent binding of purged buffer objects")
v2:
  - Use early return for imported BOs in all four helpers to avoid
    nesting (Matt B).
  - Group purgeability state into a purgeable sub-struct on struct
    xe_bo (Matt B).
  - Reword xe_bo_willneed_put_locked() kernel-doc to explain that a 1->0
    transition means all remaining active VMAs are DONTNEED (Matt B).

v3:
  - Move DONTNEED/PURGED reject from vma_lock_and_validate() into
    xe_vma_create(), gated on attr->purgeable_state == WILLNEED.
    Fixes vm_bind bypass and partial-unbind rejection on DONTNEED
    BOs (Matt B).
  - Drop .check_purged from MAP and REMAP; keep it for PREFETCH and
    add a comment why (Matt B).
  - Skip BO validation in vma_lock_and_validate() for non-WILLNEED
    VMA remnants so cleanup/remap paths do not repopulate
    DONTNEED/PURGED BOs.

Suggested-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260506132027.2556046-1-arvind.yadav@intel.com
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
(cherry picked from commit 23fb2ea56cb4fa2587bc072b04e4e698687a48e4)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>